Some Creative Destruction on a Cosmic Scale

Scientists Say Asteroid Blasts, Once Thought Apocalyptic, Fostered Life on Earth by Carrying Water and Protective Greenhouse Gas

By

Robert Lee Hotz

Updated Aug. 14, 2009 11:59 p.m. ET

In a paradox of creation, new evidence suggests that devastating avalanches of cosmic debris may have fostered life on Earth, not annihilated it. If so, life on our planet may be older than scientists previously thought -- and more persistent.

Astronomers world-wide have been transfixed by a roiling gash the size of Earth in the atmosphere of Jupiter, caused by an errant comet or asteroid that smashed into the gas giant last month. The lingering turbulence is an echo of a cataclysmic bombardment that shaped the origin of life here 3.9 billion years ago, when millions of asteroids, comets and meteors pummeled our planet.

New evidence is changing our understanding of Earth's origins. Known as the Late Heavy Bombardment, researchers say the showers of cosmic rock that pummeled Earth 3.9 billion years ago may have fostered life on Earth, not annihilated it. WSJ's Robert Lee Hotz reports.

Known as the Late Heavy Bombardment, these intense showers of rubble created conditions so hellish that scientists named this opening chapter of Earth's formation the Hadean era, after classical visions of the underworld and the realm of the dead. "The impact that killed the dinosaurs was just a firecracker compared to the impacts during this bombardment," says planetary scientist Oleg Abramov at the University of Colorado at Boulder. "If you were standing on the surface, you would have been vaporized."

Until recently, many researchers thought that this rain of rocks, lasting 20 million years or more, almost certainly wiped out early life on Earth -- perhaps more than once. No one knows. The earliest known traces of life belong to a period shortly after the asteroid showers slackened. "The idea was that we were hit so many times and so hard that, if there had been any life forming then, it would have been wiped out and required to rise again," says astrobiologist Lynn Rothschild at NASA's Ames Research Center in Mountain View, Calif.

But in their super-heated plunge through the atmosphere, these asteroids and meteors may have helped create conditions ideal for emerging life. "Everyone focuses on the meteor that hits the ground," says geochemist Richard Court at London's Imperial College. "No one thinks about the products of its journey that get pumped into the atmosphere."

As they vented, they collectively could have imported billions of tons of life-sustaining water into the air every year, Dr. Court and his colleague Mark Sephton recently determined. They calculated that these showers of volatile rocks delivered 10 times the daily outflow of the Mississippi River every year for 20 million years. By analyzing the fumes emitted under such extreme heat, they discovered these rocks also could have injected billions of tons of carbon dioxide into the air every year.

Combined with so much water vapor, the carbon dioxide could have induced a global greenhouse effect. That could have kept any life emerging on Earth safely in a planetary incubator at a time when the planet might easily have frozen because the Sun radiated 25% less energy than today. "The amount of CO2 that was produced is about the same we produce today through fossil fuel use and we know that is a climate-changing volume," says Dr. Court.

They analyzed gases emitted by 12 meteorites of the sort believed to have hit during the bombardment using a new laboratory technique called pyrolysis-FTIR, which can instantly heat samples to 1,000 degrees Celsius. They found that, on average, each meteorite could release up to 12% of its mass as water vapor and 6% as carbon dioxide.

To study so many ruinous impacts, Dr. Abramov and Stephen Mojzsis at the University of Colorado developed a global computer simulation to gauge temperatures beneath individual impact craters -- some caused by asteroids 50 miles or more in diameter.

By their calculations, our planet may have fared better than expected. Less than 25% of Earth's crust would have melted during such a bombardment. "What we find is that under no circumstances can we sterilize the Earth during the bombardment," says Dr. Mojzsis. "The surface zone was certainly sterile, but that is not where all life is."

ENLARGE

In fact, evolving microbes of the sort considered ancestral to all life forms today may have flourished underground in water heated by the impacts. Such habitable havens actually expanded during the bombardment, the computer simulation showed. Microbes able to live at temperatures ranging from 175 degrees to 230 degrees Fahrenheit could have survived unscathed. Some bacteria today thrive in even hotter water, such as those in hydrothermal vents at Yellowstone National Park.

No one knows what caused the bombardment. Nothing like it has happened since. But a controversial new perspective on orbital mechanics and the formation of the solar system suggests that Jupiter may have been partly responsible. The theory was developed by physicists at the Observatoire de la C&ocirc;te d'Azur in France, the Universidade Federal do Rio de Janeiro in Brazil and the Southwest Research Institute in Boulder, Colo.

In their hypothesis, Neptune and Uranus originally orbited much closer to the Sun. Starting about four billion years ago, Jupiter and Saturn pushed them into the more distant orbits they follow today. Indeed, Neptune may have started closer to the Sun than Uranus, but ended up farther away. Disrupting the gravitational balance, these huge planets triggered a shotgun blast of planetary buckshot so violent that Mars, Mercury and the Moon still bear its scars.

"It is literally a revolution in our ideas about how our solar system evolved," says asteroid expert William Bottke at the Southwest Research Institute. "It could be that our form of life today -- every living thing that we see today -- is due to this bombardment that happened 3.9 billion years ago."

Earth still speeds through fields of rubble and star dust. This past week, the annual Perseid meteor shower peppered the planet with hundreds of meteors per hour. Every year, 40,000 tons or so of extraterrestrial dust and debris falls on Earth -- a sprinkle compared with the millions of rocks still sheltered in the Asteroid Belt or the more distant Kuiper Belt and Oort Cloud.

"The object that hit Jupiter is not out of the ordinary for what we currently have in the solar system," says Amy Simon-Miller, chief of the planetary systems laboratory at NASA's Goddard Space Flight Center in Maryland. "From our perspective, the ones we worry about are the ones that cross the path of Earth."

So far, astronomers have discovered 784 asteroids a half mile or so in diameter that intersect Earth's orbit. They are tracking thousands of smaller ones and are searching for more. Despite close calls and false alarms, none of them so far threaten Earth, says comet expert Donald Yeomans, manager of NASA's Near-Earth Object Program Office.

In a report released Wednesday, a panel of experts convened by the U.S. National Research Council warned that Earth could still be blindsided. They are studying ways to safely deflect any that do come too close.

In this game of orbital roulette, Dr. Yeomans does have his eye on one large near-Earth asteroid called Apophis. On its next close approach past Earth, there is a 1-in-45,000 chance that the interplay of gravitational forces could nudge it onto a potential collision course.

"In the unlikely event that happens, it will come back and hit us on April 13, 2036," Dr. Yeomans says. "That's Easter Sunday."

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